Everyday life is carried on without much awareness of electricity. Since electricity is widely used as a source of energy in various fields, such as information processing, communication, and the like, it is indispensable in modern life.
Since electrical energy is important and useful, any failure in appropriate management and use thereof will possibly result in accidents such as short circuit-caused fires, electrocution, etc.
For example, a leakage current that will possibly cause such a serious accident is deeply linked with poor insulation of a circuit or device through which an electric current flows. However, checking for a leakage current takes a very long time and needs a momentary interruption of the power supply, and the numerical value of electric current corresponding to poor insulation has to be measured by an insulation resistance tester.
These days computers are used widely in society. In intelligent buildings and factory automation (FA), computer systems are continuously running day and night. Such computer systems should be checked for any leakage current while continuing to operate, that is, without being turned off even for a very short time.
Therefore, the present highly sophisticated information society requires maintenance of an uninterruptible power supply system. On this account, the insulation management of circuits and devices, through which an electric current flows, has been shifted from the conventional method of checking for a leakage current by the insulation resistance tester with power interruption, to a leakage current measurement which can be done with no power interruption. For this insulation management, there has been proposed a variety of methods of keeping power supply during measurement of a leakage current by a leak current breaker, earth leakage fire alarm or the like (as in Japanese Unexamined Patent Application Publication No. 2001-215247 and 2002-98729).
Note here that the leakage currents (I) include a leakage current (Igc) caused by earth capacitance and leakage current (Igr) caused by an earth insulation resistance involved directly with an insulation resistance. Since the above-mentioned short circuit-caused fire arises from insulation resistance, if the leakage current (Igr) alone, that arises from the insulation resistance, can accurately be detected in a circuit, it is possible to check the insulation state of the circuit and thus prevent a catastrophe such as a short circuit-caused fire or the like.